The production of cores comprising cellulosic fibrous sheets and a thermosetting resin composition useful in supporting decorative layers in the formation of high pressure decorative thermoset laminates is well known. Conventionally, said cores comprise a plurality, i.e. about 2-10, of paper core sheets impregnated with a liquid thermosetting resin composition, said core sheets being prepared by treating a web of paper, prepared by a wet-laying process, with a solution or dispersion of a thermosetting resin composition in a volatile solvent, drying said treated web to reduce the volatile matter content to a desired level and cutting said treated, dried paper web into sheets of the required dimensions.
In order to provide satisfactory handling and usage properties in said laminates, they are conventionally produced in thicknesses of from about 0.5 mm to about 2.0 mm, this thickness range being achieved primarily by superimposing the required number of said paper core sheets. While it is recognized that it would clearly be advantageous to use a single sheet to provide the core for the laminate, problems of manufacture and processing associated with the production and resin-impregnation of wet-laid paper sheets having a basis weight significantly greater than about 250 gsm (grams per square meter) and a thickness of greater than about 0.20 mm has prevented their production.
Further, for environmental and energy conservation reasons, it is considered desirable to obviate the drying stage necessary with conventionally produced resin composition treated paper cores. Attempts made in the past to avoid this drying step by providing a wet-laid paper containing a thermo-setting resin composition in solid particulate form as a sheet of the laminate core and formed during the paper making process from an aqueous slurry comprising the paper fibers and the particulate resin failed because of problems arising from the propensity of the liquid phase to convey the resin particles through the forming wire.
Wet-laid papers, while generally producing high pressure decorative laminates of excellent properties, have a propensity to cause stress-cracking of laminates under conditions of low relative humidity. Therefore, conventional high pressure decorative laminates, after a period of time well within their expected life-times, oftimes undergo a marked deterioration in their aesthetic appearance and utility. Conventional high pressure decorative laminates therefore have not always proven useful in areas where low relative humidity is a prevalent condition especially where the laminates are first subjected to modification such as by notching, cutting or other treatment whereby sharp corners are cut into their cross-section.
Wet-laid core papers also exhibit a variation in at least some of their physical properties depending upon whether the properties are measured in the direction of travel of the machine wire upon which the paper was formed, or transversely of it. This variation in properties is due to the non-random orientation of the fibers in the paper due to the alignment of fiber caused by the flow of the liquid phase onto and through the wire and by surface tension effects. Laminates produced from cores comprising said wet-laid papers also exhibit this direction dependent variation in at least some of their physical properties and although this is not generally disadvantageous, there are some applications where a laminate exhibiting less variation in physical properties according to the direction of measurement is preferred.
Recently, it has been found that high pressure decorative laminate produced from a thermosetting resin containing fibrous cellulosic core wherein the disadvantages of laminates made by conventional processes are overcome or diminished may be produced by using, as the core, an air-laid web comprising both cellulosic fibers and a thermosetting resin.
High pressure decorative laminates made from air-laid webs exhibit a toughness superior to laminates which contain, as their core, a plurality of thermosetting resin impregnated Kraft paper sheets. This toughness is evidenced by the laminates' increased resistance to stress-cracking. Furthermore, such high pressure decorative laminates containing an air-laid core also exhibit substantially equivalent uniform strength and dimensional properties regardless of the machine direction from which the measurement is taken.
Air-laid fibrous webs are prepared by disintegrating fibrous, cellulosic material into its component fibers, transporting the fibers to a foraminous moving web-forming surface and depositing the fibers thereon to form a layer with the aid of suction applied to the under side of the web-forming surface. The fibrous, cellulosic material is disintegrated into its component fibers by a machine such as a hammermill or disc refiner and the individual fibers are transported to the forming surface in an air-stream. Binder material is applied to or admixed with the fibers as a particulate solid or as a liquid spray and the web deposited therefrom is then consolidated between nip rollers. When the binder is added as a solid to the air-fiber stream, it may be introduced into the hammermill or thereafter, but before deposition on the forming surface. Additionally, when the binder is used as a spray, the sprayed fibers may thereafter be dried and introduced as such into the forming apparatus.
A known apparatus for forming substrates by air-laying cellulosic fibers comprises: (i) an air-swept hammermill wherein cellulosic material is defibrated into its component fibers in an air-stream, (ii) ducting whereby the fiber containing air-stream is conveyed to a distributor, (iii) a distributor such as disclosed in U.S. Pat. No. 3,581,706, comprising a housing having a perforated planar bottom wall and side walls, one or more impellers mounted to rotate about an axis substantially perpendicular to the bottom wall a short distance above and in non-contacting relationship with the upper surface of said bottom wall, inlet means for the fiber containing air-stream to enter the distributor, outlet means whereby fibrous material may be recycled to the hammermill and, optionally, a plate member located above said impellers and extending inwardly from the side walls of the housing so as to form a partition between a lower part and an upper part of said housing, said distributor being positioned so that the bottom wall is cooperatively located above the upper surface of (iv) a moving, foraminous belt upon the upper surface of which the cellulosic fibers are deposited to form a layer with the aid of (v) means for applying suction to the under surface of said belt and (vi) means for compacting the so-deposited cellulosic fiber layer, see U.S. Pat. Nos. 2,698,271 and 4,014,635.
When apparatus of the type described above is used in the production of an air-laid cellulosic fibrous layer, there are a large number of variables that must be controlled in order that optimum formation of the layer occurs. These variables include the input rate of the cellulosic material to the hammermill, the speed of rotation of the impellers and speed of travel of the belt and the degree of compaction applied. When preparing a core adapted for use in the production of high pressure decorative laminates, the thermosetting resin must be uniformly distributed throughout the deposited layer and there must be sufficient of the resin present to provide the desired properties to the heat and pressure consolidated laminate. In the production of such high pressure decorative laminates, the resin content of the core lies in the range from about 10% to about 40% by weight, preferably from about 25% to about 30%, based on the total weight of the core.
Additionally, for the formation of an air-laid core having the desired uniformity of composition and basis weight and comprising fibers and thermosetting resin, such as by means of an apparatus of the type described above, it is preferable to operate under conditions such that the air has a relative humidity within the range of about 40% to 80%, preferably about 50% to 70% in order to prevent deposition problems which may arise in that at too high a humidity, clogging of the ducting and screen may occur, while at too low a humidity, problems may arise due to static electrical charges on the fibers.
In the production of decorative laminates from either wet-laid or air-laid cores various forms of waste material are produced in the finishing operations necessary in connection with manufacture of these decorative laminates. One of the operations performed upon the finished laminates i.e. those recovered upon removal thereof from the laminating press, consists of trimming all the peripheral edges of each laminate in order to remove "flash", i.e. any portion of the laminate periphery which is not fully consolidated during lamination. This material is very brittle and detracts from the overall appearance of the laminate. A second operation performed upon the finished laminates is the sanding of the back or reverse side thereof so as to provide a good surface for bonding of the laminate to a suitable underlayment via the use of a contact adhesive. Also, the finished laminates are oftimes cut to size in order to supply the ultimate consumer with a variety of lengths and widths for fabrication into commercial articles, resulting in cutting dust.
The waste or offal produced by the trimming, sanding and cutting operations is a major concern to laminate manufacturers because during these finishing operations as much as 14% of the weight of the final laminate may be removed. Thus, the waste created can cause significant handling, disposal, environmental and energy concerns.
Additionally, other forms of waste accrue. That is to say, during the course of producing the components which go into the ultimate laminates i.e. the overlay sheets, the decor sheets etc., objectionable or sub-quality material may be produced which is not suitable for producing a quality laminate and therefore must be discarded. Additionally, other waste material may be generated during the laminating operation per se in the form of imperfect laminates, i.e. laminates with physical deficiencies, surface imperfections and the like.
If a use for these above-described waste materials could be found such that even a portion of the material usually discarded, burned or otherwise disposed of could be reused, a long felt need in the laminating industry would be solved.